(defun rotate (m) (apply #'map 'list #'vector (reverse m)))

(defun merge-line (&rest lines)
  (apply #'map 'string
         (lambda (&rest chars)
           (or (find #\Space chars :test #'char/=)
               #\Space))
         lines))

(defun merge-matrix (&rest matrices)
  (apply #'mapcar #'merge-line matrices))

(defun write-cross (s)
  (let* ((m (let* ((n (length s))
                   (empty (format nil "~V@T~V@T~V@T " n n n))
                   (line1 (format nil "~V@T~A~V@T " n s n))
                   (line2 (format nil "~A~V@T~A " s n s)))
              `(,line1 ,@(make-list (1- n) :initial-element empty)
                ,line2 ,@(make-list (* 2 n) :initial-element empty))))
         (m1 (rotate m))
         (m2 (rotate m1))
         (m3 (rotate m2)))
    (mapc #'write-line (merge-matrix m m1 m2 m3))))